In certain thermal printers, a thermal head having a linear array of individually energizable, e.g., resistive, heating elements is modulated (energized) to transfer heat transferable (e.g., thermally diffusible or sublimable) image imparting substance such as dye from a donor web (dye web, carrier) to an image receiver (sheet, medium) such as recording paper, for borderless printing, i.e., to print images across the full width of the receiver from one side edge to the other, by known technique.
The dye is imparted from the web to the receiver as image pixels, e.g., of 0.003 inch length and 0.003 inch height, by action of the individual heating elements. Each pixel constitutes a dye deposit on the receiver at a pixel position corresponding to a heating element. The density (darkness) of a printed dye pixel on the receiver is a function of the temperature of the heating element and the time it heats the web. Since the heat delivered by a heating element to the web causes dye transfer to the receiver as an image pixel, the dye amount transferred as a pixel is directly related to the heating element energy amount delivered to the web. The operation is controlled to achieve uniform print density.
Typically, a rotatable platen (drum) forms a printing nip with the head through which the web and receiver travel in unison as the platen rotates while the head remains stationary. The platen supports the receiver and the web is situated between the receiver and head. The head urges the web and receiver against the platen under mechanical contact force during printing for efficient dye transfer from the web to the receiver. The contact force must be uniform to avoid variation in the energy delivered by the heating elements to the web as this causes print density non-uniformity.
The linear array of heating elements defines a head printing width which for borderless printing should exceed the receiver width so as to overlap the receiver side edges to assure printing of the full receiver width in each printed line, i.e., without leaving unprinted side borders on the line. For the same reason, the web width should exceed the receiver width. The platen width should also exceed the receiver width for adequate support of the receiver. Whether the platen width exceeds, equals or is exceeded by the head printing width, the platen should form a printing nip with the head that defines a nip width along their common extent likewise exceeding the receiver width.
The individual heating elements are energized under control of borderless printing software in known manner for dye transfer from the web to the receiver as image pixels so as to assure that the pixels are imparted selectively throughout each line of printing across the width of the receiver at every pixel position from the first pixel position at its left edge to the last pixel position at its right edge. The software must control the borderless printing within precise width limits to avoid leaving an unprinted border of even a single pixel at either side edge of the receiver.
The software should also avoid overprinting at the receiver side edges, i e., printing one or more pixels on the adjacent portions of the platen. Normally, the software precisely repeatably controls the start of a printing line at the first pixel position at the receiver left edge. However, precisely repeatable stopping of the printing at the last pixel position at the receiver right edge usually requires special software for "a stop printing line" control. Use of less elaborate software that effects platen overprinting to assure borderless receiver printing, objectionably contaminates the platen with pixel dye deposits.
Aside from the basic drawback of dye contamination build up on the platen, upon changing to a wider receiver, the dye deposits around the platen circumference in the vicinity of the narrower receiver side edges will lie under the wider receiver side edges, making them uneven and the printing nip non-uniform. The deposits are random solid masses of heat transferable dye, i.e., thermally diffusible or sublimable substance, unlike liquid ink which spreads as an even film under capillary and surface tension forces.
The contaminated platen surface changes locally in nip distance from the heating elements along the nip Width and around the platen circumference in the vicinity of the randomly deposited dye pixels. This local nonuniformity disturbs the uniformity of the mechanical contact force between the head and the web and receiver and thus the uniformity of the dye amount transferred as a function of the energy delivered by the heating elements under such contact force. The density of the printed dye image pixels is thereby rendered non-uniform at the receiver side edges.
Even where elaborate software control of borderless printing is used with an arrangement in which the printing nip and receiver, and possibly also the web, have the same width, to avoid platen overprinting, the dimensional tolerances of such widths are such that physical misalignments can occur. These misalignments can cause platen overprinting despite elaborate software use.
As the head is usually a permanent part of the printer, when switching from a narrower to a wider receiver, the overprinted platen must be replaced to avoid the above problems. The fresh platen must form with the head a printing nip of proper width, as defined by their common extent, to accommodate the wider receiver, i.e., the new nip width must equal or exceed the receiver width. Taping the platen circumference adjacent the receiver side edges with masking tape to offset the overprinting problem, instead of replacing the platen, merely introduces a further source of dimensional non-uniformity and unevenness at the printing nip.
Another problem is that the platen dye deposits can build to a height sufficient under the mechanical contact force at the nip to disturb the functioning of the adjacent heating elements. If the nip width exceeds the web width, dye deposits on the rotating platen can wipe against the heating elements to contaminate and possibly misalign them, and render their efficiency nonuniform. If the web width equals or exceeds the nip width, similar action can occur through the web. If the receiver is replaced by a wider one, like action can occur through both the receiver and web.
Current borderless printing practice is thus relegated to enduring the non-uniformity problems of platen overprinting, or of platen replacement or taping to offset such problems. Of course, on switching from one width receiver to another, the software must be changed to control the new borderless printing width.
In certain single color thermal printers, a single color dye web is used to print image pixels on the receiver. In certain multicolor printers, the web has a repeating series of successive dye areas of different colors, e.g., yellow, magenta and cyan, and the receiver is conducted repeatedly past the head to transfer dye from each color area of the series in turn to the same print area of the receiver, i.e., on reregistering it each time with the head.
Various wet printer, masking device and thermal printer arrangements are known. Examples of such arrangements are shown in the following prior art.
British Patent No. 1,655 (Godchaux & Cie.) discloses a printer using liquid ink for wet printing of both sides of a sheet. On passing the sheet between a first inking roller and first pressure roller to print its first side, it is passed between a second inking roller and second pressure roller to print its second side. An ink absorbing web is located between the second pressure roller and printed first side of the sheet to prevent the ink on the first side from wetting the second pressure roller.
U.S. Pat. No. 849,454 (Beeken) discloses a printer using liquid ink for wet printing of both sides of a sheet by separate inking rollers at diametrically opposed parts of a blanket covered pressure roller. The sheet passes along one part of the pressure roller to print its first side by a first inking roller, and then is twisted and passed along the other pressure roller part to print its second side by a second inking roller. A roll-tympan web is located between the blanket and sheet to receive the offset (ink) from the printed first side of the sheet.
British Patent No. 235,545 (Koechlin S. A.) discloses a printer using liquid ink for wet printing of fabric passed between an inking roller and pressure roller. The fabric is separated from the inking roller by an endless cloth band and a wire gauze band.
U.S. Pat. Nos. 1,287,524 and 1,700,865 (Trier) disclose a printer using liquid ink for wet printing of both sides of a sheet. After printing the first side, the second side is printed by passing the sheet between an inking roller and pressure roller covered by a web to receive the offset (ink) from its first side.
U.S. Pat. No. 1,873,207 (Knowlton) discloses a printer using liquid ink for wet printing of both sides of a sheet. A cover or belt on a pressure roller receives the offset (ink) from the first side of the sheet during printing of its second side.
U.S. Pat. No. 2,175,051 (Bromley) discloses a printer using liquid ink for wet printing of fabric passed between an inking roller and pressure roller. The fabric is separated from the pressure roller by an inner blanket and an outer blotting web.
British Patent Specification No. 715,021 (Verduin) discloses a printer using wet ink for wet printing of fabric passed between multiple color stations and a single pressure roller. The fabric is separated from the pressure roller by a blanket.
U.S. Pat. No. 4,478,878 (Neuwald) discloses the forming of a metal-free strip on insulating tape used in an electrical capacitor. A strip area on the insulating tape is covered by masking tape, a metal coating is deposited thereon, and then the masking tape is removed to leave the metal-free strip.
U.S. Pat. No. 4,571,102 (Ono et al.) discloses an apertured masking frame for mounting a dot matrix printer ribbon.
U.S. Pat. No. 4,904,098 (Hamilton) discloses an apertured flexible masking shield for the print wheel of an impact printer.
U.S. Pat. No. 4,929,102 (Mizutani) discloses an apertured mask for the ribbon of a mechanical print head pin type printer.
U.S. Pat. No. 4,919,555 (Kikuchi) discloses a thermal printer with adjustable axis mounting means used to print adhesive backed labels carried on a support sheet traveling in unison with a carbon printing ribbon through the printing nip between the thermal head and platen (drum). In the embodiment shown, the head, platen, printing nip, ribbon, labels and support sheet, all have the same width. Borderless printing is not contemplated.
It is desirable to provide a thermal printing assembly having a thermal head and platen defining a printing nip for borderless printing of different width image receivers using a corresponding donor web, while preventing lateral overprinting on the platen, without the need for elaborate or special software.